Add shading ink (for indexed images only) and color swatches (no UI yet)
Separate ui::ListBox::Item into ui::ListItem class
When a widget is removed (through Widget::removeChild) we've to remove it from its ui::Manager

A widget without parent, cannot be part of the Manager (e.g. cannot be
the capture widget, or focused, or the widget with mouse).
Use ui::ListItem in ui::ComboBox widget
Fix bug in import sprite sheet

The editor must be released before we destroy the document. (The document
destroys all its views now.)
Add support for transparent background color for ui::Slider to draw ui::IntEntry popup window without background